Transcript Document
Competition – Chapter 12 Competition • Resource competition – occurs when a number of organisms utilize common resources that are in short supply. • Interference competition – occurs when the organisms seeking a resource harm one another in the process, even if the resource is not in short supply. • Interspecific competition – competition between two or more different species. – Owls and hawks competing against each other for the same food source • Intraspecific competition – competition between members of the same species. – Owls competing among themselves for the same food source 1 dN1 K1 - N1 = r1N1 dt K1 1 2 2 2 2 1 1 dN2 K2 – N2 = r2N2 dt K2 1 2 Square size = amount of resource needed by an individual 2 2 Because an individual of species 2 does not use the same amount of resource as an individual of species 1, we use an equivalent number of species 1 individuals = N2. We can write the competition equation for species 1 as: dN1 K1 - N1 - N2 = r1N1 dt K 1 N2 accounts for species 2 For the same reason, we use an equivalent number of species 2 individuals = N1. We can write the competition equation for species 2 as: dN2 K2 – N2 - N1 dt = r2N2 K2 The presence of a competitor reduces the carrying capacity. • Competitive Exclusion Principle – complete competitors cannot coexist. – One species must be displaced or go to extinction • Fundamental niche – the set of resources a species can utilize in the absence of competition and other biotic interactions. • Realized niche – the observed resource use of a species in the presence of competition. – Realized niche may be found on the edge of the fundamental niche as a result of competitive exclusion • Does competitive exclusion occur in natural communities? Situations Where Competitive Exclusion Would Not Occur 1) Unstable environments that never reach equilibrium and are occupied by colonizing species 2) Environments in which species do not compete for resources 3) Fluctuating environments that reverse the direction of competition before extinction is possible Competition in Nature • Competition in nature is rare • Competition has been very common throughout the evolutionary history of communities and has resulted in adaptations that serve to minimize competitive effects – Species have evolved to reduce competition • We currently see the results of competition • Invasive species? • Species that come into competition may evolve differences to minimize the impact of competition: Species can live together Some resources are not being used, and a benefit can be gained by utilizing that resource. Species may be driven to extinction, change habitats, or evolve feeding differences. Offshore Largest Nearshore Evidence for competition causing different resource utilization. Smallest Criteria for listing the occurrence of interspecific competition: Criteria 1. Observed checkerboard patterns of distribution consistent with predictions 2. Species overlap in resource use Strength of evidence Weak Suggestive 3. Intraspecific competition occurs 4. Resource use by one species reduces availability to another species 5. One or more species is negatively affected 6. Alternative process hypotheses are not consistent with patterns Convincing Evolution of Competitive Ability • If two species are competing, both would benefit by evolving differences to reduce the competition. • However, if a species evolves to utilize a smaller food source, it may face other competitors. • Thus a species may evolve competitive ability. – Theory of r-selection and K-selection in animals – C-S-R theory in plants r-selction and K-selection • r (rate of increase) and K (carrying capacity) characterize the logistic curve – Can characterize organisms by the relative importance of r and K in their lives • In some environments, organisms exist near their asymptotic density (K) for much of the year. – These organisms are subject to K-selection • In other habitats, the same organisms may rarely approach the asymptotic density but instead remain on the rising portion of the curve for most of the year. – These organisms are subjected to r-selection r-selction and K-selection • Species that are r-selected seldom suffer from much pressure from interspecific competition. – Hence, they evolve no mechanisms for strong competitive ability • Species that are K-selected exist under both intra- and interspecific competition. – Organisms are pushed to use their resources more efficiently (or ) - selection • Species can evolve competitive ability by the process of -selection. – equivalent number of species 1 individuals = N2. – By increasing competitive ability, each individual of species 2 has a larger impact on species 1 • Usually leads to interference competition – Territorial behavior in animals – Allelopathic chemicals in plants • Major evolutionary problem with this type of selection is that you are competing with members of your own species. Idealized Evolutionary Gradient Low Density – colonization and growth (r selection) High Density – resource competition (K selection) High Density – interference mechanisms ( selection) prevent resource competition Grime’s Theory of Plant Strategies • Stress – physical-chemical limitations (shortages of resources) – Light, water, nitrogen, temperature stresses • Disturbance – events such as grazing, diseases, wind storms, frost, erosion, and fire Intensity of stress Intensity of disturbance Low High Low Competitive (K) strategy Stress-tolerant strategy High Ruderal (weed or r) strategy None possible Character Displacement • Divergence of species in areas where they occur together Occur Together Occur Separate